The present invention relates generally to delivery systems for delivering and deploying medical devices, such as stents, which are adapted to be implanted into a patient's body, such as a blood vessel and, more particularly, to a delivery system for more accurately deploying a self-expanding medical device into an area of treatment.
Stents are generally cylindrically shaped devices which function to hold open and sometimes expand a segment of a blood vessel or other arterial lumen, such as coronary artery. Stents are usually delivered in a compressed condition to the target site and then deployed at that location into an expanded condition to support the vessel and help maintain it in an open position. They are particularly suitable for use to support and hold back a dissected arterial lining which can occlude the fluid passageway there through. Stents are particularly useful in the treatment and repair of blood vessels after a stenosis has been compressed by percutaneous transluminal coronary angioplasty (PTCA), percutaneous transluminal angioplasty (PTA), or removed by atherectomy or other means, to help improve the results of the procedure and reduce the possibility of restenosis. Stents, or stent-like devices, are often used as the support and mounting structure for implantable vascular grafts which can be used to create an artificial conduit to bypass the diseased portion of the vasculature, such as an abdominal aortic aneurism.
A variety of devices are known in the art for use as stents and have included coiled wires in a variety of patterns that are expanded after being placed intraluminally on a balloon catheter; helically wound coiled springs manufactured from an expandable heat sensitive metal; and self-expanding stents inserted into a compressed state for deployment into a body lumen. One of the difficulties encountered in using prior art stents involve maintaining the radial rigidity needed to hold open a body lumen while at the same time maintaining the longitudinal flexibility of the stent to facilitate its delivery and accommodate the often tortuous path of the body lumen.
Prior art stents typically fall into two general categories of construction. The first type of stent is expandable upon application of a controlled force, often through the inflation of the balloon portion of a dilatation catheter which, upon inflation of the balloon or other expansion means, expands the compressed stent to a larger diameter to be left in place within the artery at the target site. The second type of stent is a self-expanding stent formed from shape memory metals or superelastic nickel-titanium (NiTi) alloys, which will automatically expand from a compressed state when the stent is advanced out of the distal end of the delivery, or when a restraining sheath which holds the compressed stent in its delivery position is retracted to expose the stent.
Some prior art stent delivery systems for delivery and implanting self-expanding stents include an member lumen upon which the compressed or collapsed stent is mounted and an outer restraining sheath which is initially placed over the compressed stent prior to deployment. When the stent is to be deployed in the body vessel, the outer sheath is moved in relation to the inner member to “uncover” the compressed stent, allowing the stent to move to its expanded condition. Some delivery systems utilize a “push-pull” type technique in which the outer sheath is retracted while the inner member is pushed forward. Another common delivery system utilizes a simple pull-back delivery system in which the self-expanding stent is maintained in its compressed position by an outer sheath. Once the mounted stent has been moved at the desired treatment location, the outer sheath is pulled back via a deployment handle located at a remote position outside of the patient, which uncovers the stent to allow it to self-expand within the patient. Still other delivery systems use an actuating wire attached to the outer sheath. When the actuating wire is pulled to retract the outer sheath and deploy the stent, the inner member must remain stationary, preventing the stent from moving axially within the body vessel.
However, problems have been associated with such prior art delivery systems. For example, systems which rely on a “push-pull design” or “pull-back design” can experience unwanted movement of the collapsed stent within the body vessel when the inner member is pushed forward which can lead to inaccurate stent positioning. Systems which utilize a “pull back” system or an actuating wire design will tend to move to follow the radius of curvature when placed in curved anatomy of the patient. As the outer sheath member is actuated, tension in the delivery system can cause the system to straighten. As the system straightens, the position of the stent changes because the length of the catheter no longer conforms to the curvature of the anatomy. This change of the geometry of the system within the anatomy can lead to inaccurate stent positioning.
Delivery systems which utilize the “pull-back” type technique usually require removal of “slack” developed between the outer sheath and the inner catheter member upon which the stent is mounted. Generally, the exposed catheter, i.e. the portion of the outer member which remains outside of the patient, must usually be kept straight or relatively straight during deployment. Failure to do so may result in deploying the stent beyond the target area and can cause the stent to bunch up. This phenomenon occurs because the inner catheter member tends to move forward when the outer sheath is retracted. The reason why this happens is because the inner catheter member and outer catheter member are typically the same length prior to stent deployment. The length of the exposed catheter, again, the portion of the outer catheter which extends between the deployment handle and the insertion point in the patient, however, is usually fixed. When the outer sheath is retracted proximally into the deployment handle during deployment, the length of the exposed outer sheath tends to shorten. The inner catheter member, however, remains the same length as it is held fixed in the deployment handle. However, the outer sheath tends to shorten during deployment, thus changing the shape of the exposed portion of the catheter. This shape change occurs because the outer sheath wants to straighten out once it's being retracted. Since the inner catheter member is fixed proximally within the deployment handle, it will move distally as the outer sheath is retracted. As a result, the movement of the inner catheter member caused by the retraction of the outer sheath can cause the stent to deploy prematurely and at a location beyond the targeted site. As a result, less than accurate deployment of the stent can occur.
This problem usually does not exist when the delivery system is kept straight during deployment as the outer sheath is allowed to slide proximally relative to the inner catheter member. However, if the delivery system is not kept straight during deployment, then the inner catheter member has this tendency to move distally during deployment. This change in the shape of the exposed catheter forces the inner catheter member to change shape as well in order for the inner catheter member to maintain the same length as the outer sheath. Since the inner catheter member is fixed within the deployment handle, it can only move distally. Consequently, the inner catheter member moves distally along with the mounted stent, causing the stent to be deployed beyond the targeted site in the patient's anatomy.
The above-described stent delivery systems also can be somewhat difficult to operate with just one hand, unless a mechanical advantage system (such as a gear mechanism) is utilized. Often, deployment with one hand is desirable since it allows the physician to use his/her other hand to support a guiding catheter which may be utilized during the procedure. The above-described stent delivery systems should not be susceptible to any axial movement of the catheters during stent deployment. Even a slight axial movement of the catheter assembly during deployment can cause some inaccurate placement of the stent in the body lumen. Some stent delivery systems employ a control handle which utilizes a pistol grip actuator that requires the physician to repeatedly pull back a trigger mechanism to cause the outer sheath to retract. In doing so, the physician usually creates a backwards force on the delivery system which also can cause the catheter portion of the delivery system to move within the patient's vasculature, resulting in less than accurate placement of the stent within the patient. Also, some of these stent delivery systems have a limited range of retraction of the outer sheath which can limit the use of the delivery system to smaller medical devices which require only a small amount of retraction in order to expand the device. Larger medical devices, such as vasculature grafts, may not be deployed because the control handle of the system may not retract the outer sheath a sufficient length in order to expose the entire graft.
Other problems associated with stent delivery systems include the buildup of frictional forces between the contacting surfaces of the tubing utilized to form the catheter portion of the delivery system. In some stent delivery system designs, there is often a sizable length of tubing which extends co-axially within another tubing, for example, an inner tubular member utilized to mount the collapsed stent and an outer retractable sheath utilized to cover the collapsed stent until the stent is ready to be deployed. Due to the surface contact between the particular surfaces of the tubular members, both static friction and dynamic friction can develop and effect the amount of force which must be developed by the control handle of the delivery system in order to retract the outer sheath. Certainly, a physician/surgeon does not wish to utilize a control handle which requires an appreciable amount of force to be exerted in order to retract the outer sheath. The physician also desires a control handle which will provide a smooth and steady retraction motion. Unfortunately, frictional buildup can possibly effect the performance of the control handle. For these reasons, it is beneficial if the amount of friction between sliding surfaces be reduced as much as possible in order to increase performance of the delivery system.
A stent delivery system which uses rapid exchange (Rx) technology can allows for quicker handling and delivery of the delivery system over the guide wire. A Rx platform enables the physician to load the distal end of the catheter portion onto the proximal end of the guide wire using a small guide wire lumen formed at the distal end of the catheter portion. The Rx design eliminates the need for a long guide wire lumen which extends from the proximal end of the system to the distal end. Additionally, a Rx design eliminates the need to have a large length of guide wire extending outside of the patient since the guide wire lumen is relatively short in length. The Rx design also tends to allow the distal end of the catheter to be more quickly delivered over the guide wire into the area of treatment.
Thus, there is a need for a delivery system for delivering and deploying a self-expanding medical device, such a stent, which prevents the axial movement of the inner catheter member relative to the outer sheath to prevent the inner catheter member from moving forward during deployment. Such a delivery system also should compensate for any slack that may be present in the delivery system and should prevent the inner catheter member from moving forward within the patient's vasculature as the outer restraining sheath is being retracted from the self-expanding medical device. A delivery system having these features would be beneficial if it allowed the physician to actuate the system with only one hand, thus allowing the physician to use his/her other hand during the procedure. Additionally, a stent delivery system including these features in a Rx design would be highly beneficial. Also, a delivery system which reduces the amount of frictional forces generated between moving components of the catheter portion of the system should improve deployment performance. The present invention disclosed herein satisfies these and other needs.